High-frequency characterization of th~n .. fnm Y.. BaDCu oxide superconduciing transmission Unes

Recently, some of us 1.2 and others' have reported on the propagation of ultrafast transients on ordinary supercon­ ducting transmission lines, since it is believed that these ma­ terials have the potential for distortion-free signal propaga­ tion into the terahertz regime. For an ordinary Bardeen­ Cooper-Schrieffer (BCS) type superconductor the energy­ gap frequency at zero temperature, fG (0) is related to the transition temperature Tc by fG (0) = 3.52 kB Tc1h, where ks is Boltzmann's constant, h is Planck's constant, and 3.52 is a constant which varies from material to material. One can easily see that for BeS type behavior the above equation gives a value ofthe energy-gap frequency in the tens oJterahertz for transition temperatures of :::;90 K. This tran­ sition temperature is now routinely achieved in the new YBa2Cu,07 x (YBCO) ceramic 4 although the exact nature of the coupling mechanism for the YBCO material is still controversial. Studies ofthe high-frequency properties ofYBCO mate­ rials, based on response of the superconductors to micro­ wave frequency electromagnetic perturbations, are of special interest, because they can give insight into the exact nature of the energy gap, quasiparticle excitation, and the pairing mechanism. From the applications point of view, simple ex­ trapolation of the Mattis-Bardeen theory predicts, for ex­ ample, distortion-free propagation of electrical pulses hav­ ing bandwidths of tens of terahertz. The first published experiments S • 6 were performed on bulk, ceramic samples and showed reasonable agreement with the BCS theory in the very dirty limit.5 However, the results were very strongly influenced by the granular, not funy controlled nature of the tested samples. 6 Workers at IBM recently reported results with aluminum lines and a YBCO ground plane,7 showing no absorption for the pulses propagating on the normal alu­ minum lines. In this letter we report our preliminary studies of picose­ cond electrical transients propagated on thin-film, coplanar transmission lines made out of the YBCO material. The